Nitrogen dioxide (NO2) is a gas. It can be emitted directly into the air but is most often formed when nitric oxide (NO) emissions react with other chemicals in the air. Nationally, road motor vehicles are the main human-made source of nitrogen oxides (NOx, the collective term for NO2 and NO). NO2 may cause respiratory infections and reduced lung development and functioning.

Nitrogen dioxide (NO2) and nitric oxide (NO) (collectively known as nitrogen oxides, NOx) are emitted from the combustion of fossil fuels and from natural sources such as volcanoes. The four main human-made sources of key pollutants in New Zealand are burning wood or coal for home heating, road motor vehicle use, industry, and household outdoor burning.